The present invention relates to a circuit arrangement for combining high frequency power components of the same frequency which come from amplifiers or generators and are fed to a 90.degree. parallel circuit bridge having two substantially decoupled inputs, with the input resistance values of the power components depending on the output load of the circuit bridge.
For high power transmitters it is often necessary to connect a plurality of amplifier elements, e.g., tubes, transistors or the like, in parallel in the final stage in order to obtain the desired output energy. This applies in particular for transistors since at present the maximum output power of high frequency transistors lies only in the order of magnitude of 100 W. The following comments are therefore directed to transistors although in principle they can also be transferred to other amplifier elements, e.g., tubes.
Since a direct parallel connection of transistors involves considerable difficulties and has some drawbacks, it is the custom to effect such parallel connections via parallel circuit arrangements, particularly in the form of parallel circuit bridges. In this way the transistors are decoupled from one another in the parallel connection.
Two groups of parallel circuit bridges are known: bridges with inputs which are fed in phase (or shifted in phase by 180.degree.) and bridges which are fed with a 90.degree. shift in phase (90.degree. bridges). The 90.degree. bridges, usually as so-called 3 dB couplers in the form of 2 .times. .lambda./4 bridges with coupled lines (where .lambda. = wavelength), are employed most often.
In a 90.degree. bridge, different input resistances will appear at the inputs if the load resistance or the antenna are mismatched and this has the result that the generators to be connected in parallel, i.e., the generators or amplifiers whose outputs are connected to the inputs of the 90.degree. bridge, receive different terminating resistances. For one of the generators its terminating resistance will appear in inverted form compared to the other generator.
For high frequency power amplifiers in class C operation the output power P.sub.o is approximately ##EQU1## where V.sub.CE is the collector voltage and
R is the collector load resistance of the output stage transistor.
The output power P.sub.o is thus inversely proportional to the load resistance R. With the same direct collector voltage for both generators (or amplifiers) to be connected in parallel, a mismatched 90.degree. bridge would cause the two generators to emit different powers because, due to the mismatch, R would be different for both generators. With greater mismatches the difference becomes considerable. In order to make such mismatches possible without endangering the amplifying elements, the latter must be given sufficient dimensions with respect to their maximum permissible power loss. This considerably increases the costs for the prior art transmitter output stages.
A further drawback of a mismatched 90.degree. bridge is that the efficiency of the transmitter is poorer when compared to proper matching because the output power P.sub.s of the transmitter with a mismatch drops to ##EQU2##
Here again, as above, P.sub.o is the output power with matching. The difference in power flows as loss energy into the dummy resistor at the output of the bridge in which no power is destroyed if there is matching.